The proposed research will investigate the feasibility of using intra-operative ultrasound (US) images to non-invasively register the bone surfaces of the knee to preoperative magnetic resonance (MR) images. A surgical navigation system, KneeNav, is being developed by CASurgica for intraoperatively planning the proper location of ligament attachment sites and drill tunnel locations and then guiding the surgeon to execute the plan. Despite agreement on the correct points of insertion, great variability exists in tunnel placement among surgeons and the rate of misplaced tunnels in ACL reconstruction surgery has been reported to be between 10-40 percent. Malpositioning of the bone tunnels is the main reason for revision surgery. The proposed research would increase the accuracy of the procedure versus current videoscopic techniques or versus competitive image guidance systems. The goal of the proposed research is to continue development of a minimally invasive surgical navigation platform that relies on magnetic resonance (MR) imaging and tracked ultrasound. The project will build on the milestones achieved during Phase I, and focus on development of a surgical navigation station that would fully support the use of this newly integrated technology in the operating room. The clinical focus of the proposed project is anterior cruciate (ACL) ligament surgery. However, the technologies developed and integrated in the system will be applicable to any image guided minimally invasive surgical procedure in orthopaedics. Phase II of the proposed project will focus on the following new challenges: (1) Development of highly automated procedure for magnetic resonance (MR) image segmentation, based on statistical atlases and deformable 3D snakes; (2) Development of user interfaces to ensure reliable collection of ultrasound data for intraoperative registration; and (3) development of a navigation cart that will integrate tracked ultrasound with the optical tracking system and other standard surgical navigation hardware into a single operating room deployable unit.